Selective expression of a constitutively active erythropoietin receptor in GABAergic neurons alters hippocampal network properties without affecting cognition

Erythropoietin restrains the inhibitory potential of interneurons in the mouse hippocampus

Severe psychiatric illnesses, for instance schizophrenia, and affective diseases or autism spectrum disorders, have been associated with cognitive impairment and perturbed excitatory-inhibitory balance in the brain. Effects in juvenile mice can elucidate how erythropoietin (EPO) might aid in rectifying hippocampal transcriptional networks and synaptic structures of pyramidal lineages, conceivably explaining mitigation of neuropsychiatric diseases. An imminent conundrum is how EPO restores synapses by involving interneurons. By analyzing ~12,000 single-nuclei transcriptomic data, we generated a comprehensive molecular atlas of hippocampal interneurons, resolved into 15 interneuron subtypes. Next, we studied molecular alterations upon recombinant human (rh)EPO and saw that gene expression changes relate to synaptic structure, trans-synaptic signaling and intracellular catabolic pathways. Putative ligand-receptor interactions between pyramidal and inhibitory neurons, regulating synaptogenesis, are altered upon rhEPO. An array of in/ex vivo experiments confirms that specific interneuronal populations exhibit reduced dendritic complexity, synaptic connectivity, and changes in plasticity-related molecules. Metabolism and inhibitory potential of interneuron subgroups are compromised, leading to greater excitability of pyramidal neurons. To conclude, improvement by rhEPO of neuropsychiatric phenotypes may partly owe to restrictive control over interneurons, facilitating re-connectivity and synapse development.

Introducing the brain erythropoietin circle to explain adaptive brain hardware upgrade and improved performance

Executive functions, learning, attention, and processing speed are imperative facets of cognitive performance, affected in neuropsychiatric disorders. In clinical studies on different patient groups, recombinant human (rh) erythropoietin (EPO) lastingly improved higher cognition and reduced brain matter loss. Correspondingly, rhEPO treatment of young rodents or EPO receptor (EPOR) overexpression in pyramidal neurons caused remarkable and enduring cognitive improvement, together with enhanced hippocampal long-term potentiation. The 'brain hardware upgrade', underlying these observations, includes an EPO induced ~20% increase in pyramidal neurons and oligodendrocytes in cornu ammonis hippocampi in the absence of elevated DNA synthesis. In parallel, EPO reduces microglia numbers and dampens their activity and metabolism as prerequisites for undisturbed EPO-driven differentiation of pre-existing local neuronal precursors. These processes depend on neuronal and microglial EPOR. This novel mechanism of powerful postnatal neurogenesis, outside the classical neurogenic niches, and on-demand delivery of new cells, paralleled by dendritic spine increase, let us hypothesize a physiological procognitive role of hypoxia-induced endogenous EPO in brain, which we imitate by rhEPO treatment. Here we delineate the brain EPO circle as working model explaining adaptive 'brain hardware upgrade' and improved performance. In this fundamental regulatory circle, neuronal networks, challenged by motor-cognitive tasks, drift into transient 'functional hypoxia', thereby triggering neuronal EPO/EPOR expression.

Deficiency of a brain-specific chemokine-like molecule, SAM3, induces cardinal phenotypes of autism spectrum disorders in mice

Chemokines are small secreted signaling proteins produced by a broad range of cells, including immune cells. Several studies have recently suggested potential roles of chemokines and their receptors in the pathophysiology of autism spectrum disorders (ASDs). SAM3 is a novel brain-specific chemokine-like molecule with an unknown physiological function. We explored the relevance of chemokines in the development of ASD in mice, with a focus on SAM3. We generated Sam3 gene knockout (KO) mice and characterized their behavioral phenotypes, with a focus on those relevant to ASD. Sam3-deficient mice displayed all three core phenotypes of ASD: impaired responses to social novelty, defects in social communication, and increased repetitive behavior. In addition, they showed increased anxiety. Interestingly, gender differences were identified for several behaviors: only male Sam3 KO mice exhibited increased anxiety and increased repetitive behaviors. Sam3 KO mice did not exhibit changes in other behaviors, including locomotor activities, fear learning and memory, and object recognition memory. These findings indicate that a deficiency of SAM3, a novel brain-specific chemokine-like molecule, may lead to the pathogenesis of ASDs and suggest the possibility that SAM3, a soluble factor, could be a novel therapeutic target for ASD treatment.